1. Field of the Invention
The present invention relates to a tint block image generation program and a tint block image generation device, and more particularly to a program and device for generating tint block image data to be printed on a print medium. The present invention also relates to a tint block image generation program and generation device which has an effect to inhibit forgery by copying a print medium (original) on which a tint block image is printed based on the tint block data or an effect to distinguish between the original and the copy.
2. Description of the Related Art
The forgery inhibited tint block is combined with the original image as background, and allows distinguishing whether the print document is the original or the copy. Characters or images in the forgery inhibited tint block can hardly be identified in the original, but if copied, the characters or images in the tint block emerge. Using this, the original and the copy can easily be distinguished. Also the characters or images in the tint block emerge in copying, so if an original is generated combining with the forgery inhibited tint block, an attempt to copy the original is psychologically discouraged.
The forgery inhibited tint block is disclosed in Japanese Patent Application Laid-Open No. 2005-151456, and details follow according to this description.
Generally a forgery inhibited tint block is comprised of two areas: a “latent image portion” where dots printed in the original remain or decrease little by copying, and a “background portion” where dots printed in the original are lost or greatly decreased by copying. In other words, in the latent image portion, density changes little by copying, and the original image is reproduced as is, and in the background portion, density changes considerably by copying, and the original image disappears. The characters or images of the tint block are generated by these two areas, and the characters and images of the tint block are called the “latent image”.
The densities of the latent image portion and the background portion are roughly the same, and in the original state, it is visually difficult to find such characters or images as “COPIED” of Japanese character are concealed in the tint block, but at the micro level, the background portion and latent image portion have different characteristics. When the tint block is copied, a density difference is generated between the latent image portion and the background portion, because of the difference of the respective change of density, which makes it easier to discern the characters or images of the tint block created by these two areas.
The latent image portion is comprised of clustered dots where individual dots cluster so that dots can be easily read when copying (scanning by copying), whereas the background portion is comprised of dispersed dots so that dots cannot be easily read when copying. By this, dots tend to remain in the latent image after copying, and dots tend to disappear in the background portion more easily than the latent image portion. Clustered dots or dispersed dots can be implemented by half tone processing using a different number of lines of half tone dots. In other words, half tone dots of which screen ruling is low are used to obtain a clustered dot arrangement, and half tone dots of which screen ruling is high are used to obtain a dispersed lot arrangement.
Generally a copier has a limitation in image reproducing capability, which depends on the input resolution in a step of reading the micro dots of a copy target original by a scanner, and the output resolution in a step of reproducing micro dots, read by the scanner, using a print engine. Therefore if isolated micro dots exist in the original, exceeding the limitation of the image reproducing capability of the copier, the micro dots cannot be perfectly reproduced in a copy, and the portions of the isolated micro dots disappear. In other words, if the background portion of the forgery inhibited tint block is created so as to exceed the limitation of the dots that the copier can reproduce, then large dots (clustered dots) in the forgery inhibited tint block can be reproduced by copying, but small dots (dispersed dots) cannot be reproduced by copying, and a concealed latent image appears in the copy. Even if the dispersed dots in the background portion do not disappear completely by copying, a density difference is generated between the background portion and the latent image portion after copying if the degree of loss of dots is high, compared with the clustered dots in the latent image portion, then a concealed latent image appears in the copy.
In the forgery inhibited tint block, a technology called “camouflage” is used to make it more difficult to discern characters or images concealed as a latent image. This camouflage technology is a method for arranging patterns, of which density is different from the latent image portion and the background portion, in the entire forgery inhibited tint block image, and in a macro view, the camouflage patterns, of which density is different from the latent image portion and the background portion, standout, making the latent image even more obscure. In other words, the contrast of the camouflage patterns is high, and the contrast of the latent image portion and the background portion is smaller than this, so the latent image is more effectively concealed because of optical illusion. Also the camouflage pattern can give a decorative impression on printed matter, and allows creating an artistically designed forgery inhibited tint block. Generally a camouflage pattern is created in binary, and the camouflage pattern is formed by not generating dots of the tint block in an area corresponding to the camouflage pattern. The above is an overview of the forgery inhibited tint block.
FIG. 1 shows an example of a latent image of a forgery inhibited tint block and a camouflage pattern. In a latent image mask pattern 10 of the Japanese character “COPY”, the black portion corresponds to the latent image portion LI of the tint block, and the white portion corresponds to the background portion BI of the tint block, for example, as the enlarged view 10X shows. In the camouflage pattern 12, on the other hand, the black portion CAM becomes an area where the dots of the tint block are not formed, and the white portion becomes an area where dots of the tint block are formed, for example, as the enlarged view 12X shows. In other words, the data of the camouflage pattern is binary image data where each pixel indicates a portion to print the tint block image and a portion not to be printed.
FIG. 2 is a diagram depicting an example of an original in which a forgery inhibited tint block is printed. In the tint block 14, a latent image portion LI and a background portion BI are formed based on the latent image mask pattern 10 in FIG. 1. The latent image portion LI is formed by dots with low screen ruling (53 lpi) based on a clustered dot dither method, and the background portion BI is formed of dots with high screen ruling (212 lpi) based on the dispersed dot dither method. As the enlarged tint block 14X shows, the entire tint block has a predetermined output density, but the dots in the latent image portion LI are large dots formed by a screen with low screen ruling, and the dots in the background portion BI are small dots formed by a screen with high screen ruling.
In the tint block 16, the latent image portion LI and the background portion BI are formed, excluding a black area CAM of the camouflage pattern, based on the latent image mask pattern 10 and the camouflage pattern 12 in FIG. 1. As the enlarged tint block 16X shows, the entire tint block has a predetermined output density, where dots are not formed in the area CAM of the camouflage pattern, and in another area, the latent image portion LI formed by large dots and the background portion BI formed by micro dots are formed just like FIG. 1. Since the contrast of the camouflage pattern is high, the latent image (the Japanese character “COPY”), comprised of the latent image portion LI and the background portion BI, of which contrast is low, does not stand out.
In the original of the forgery inhibited tint block in FIG. 2, the output density of the latent image portion LI and the background portion BI are the same, whereby the latent image of the Japanese character “COPY” formed by these portions is concealed. This is referred to as the “concealment capability for a latent image in the original is high”.
FIG. 3 is a diagram depicting an example of a copy of the forgery inhibited tint block. The copy 18 is created via a scanning step and dot generation step (step of printing the print media based on the scan data generated in the scanning step) by copying, and as the enlarged view 18X shows, large dots in the latent image portion LI are hardly lost, but many micro dots in the background portion BI are lost. As a result, in the copy 18, the output density of the latent image LI hardly drop, but the output density of the background portion BI drop considerably, and the latent image of the Japanese character “COPY” emerges. In other words, the latent image of the copy is more easily identified.
The copy 20 is the same as the copy 18, except for the area CAM of the camouflage pattern. The contrast of the camouflage pattern drops because of the drop in the output density of the background portion BI, and the latent image COPY emerges.
As mentioned above, increasing the concealment capability for the latent image in the original and increasing the identification capability for a latent image in the copy are demanded for tint blocks. However, if the latent image portion of the tint block is formed by dots with low screen ruling using a clustered dot screen and the background portion is formed by dots with high screen ruling using a dispersed dot screen, as in the case of the prior art, a high concealment capability for the latent image in the original and a high identification capability for the latent image in the copy cannot be implemented.
In order to increase the identification effect of the latent image in the copy so as to increase the copy inhibition effect, increasing the output density for the latent image portion is desirable. This is because the identification capability for the latent image increases if the output density of the latent image portion which emerges in the copy increases, as shown in FIG. 3, and the copy inhibition effect increases.
However, if the output density is increased by further increasing the size of the large dots in the latent image portion, the output density of the background portion must also be increased in order to maintain the concealment capability for the latent image in the original. This is because the concealment capability for the latent image in the original is increased by making the output density of the latent image portion and the output density of the background portion the same or similar. Increasing the output density of the background portion using a conventional configuration, however, has the following problems.
First the background portion is comprised of micro dots dispersed by the dot dispersed screen, so in order to increase the output density of the background portion, the density of the micro dots must be increased by increasing the screen ruling. However the screen ruling of the background portion has a certain limitation due to the limitation of the dot reproducing capability of the printer engine, so if the screen ruling is increased beyond the limitation, dispersed dots can no longer be reproduced. In other words, in actual printed matter, the dots which were supposed to be disposed influence each other and combine, and can no longer exist as micro dots. As a result, the dots in the background portion do not disappear in the copying step, and the output density difference cannot be generated between the latent image portion and the background portion, and the identification capability in the copy drops.
Secondly, an attempt to implement the concealment capability of a latent image in the original by equalizing the output density of the background portion with that of the latent image portion, by increasing the screen ruling of the background portion to increase the density of micro dots, does not work due to the following reason. In the background portion having high screen ruling, which sensitively responds to the characteristics of the printer engine, a density unevenness in the background portion clearly appears, but in the latent image portion with low screen ruling, which is not influenced very much by the characteristics of the printer engine, the density unevenness is hardly generated.
FIG. 4 shows a tint block which has a density unevenness in the background portion, and a tint block which has a difference in hue and saturation between the background portion and the latent image portion. In FIG. 4, the tint block 22 has an area where the latent image stands out and an area where the latent image does not stand out, which coexist in the background portion, because of the above mentioned density unevenness that is generated in the background portion. The tint block 22E is the tint block 22 which is artificially exaggerated.
In the case of the forgery inhibited tint block formed by high brightness color toner, such as cyan or magenta, the difference in hue and saturation between the latent image portion and background portion further increases because of the large difference in screen rulings, and the latent image tends to stand out throughout the tint block. The tint block 24 in FIG. 4 is an example when the difference in hue and saturation increased, and the latent image stood out. The tint block 24E is the tint block 24 which is artificially exaggerated. The difference in hue and saturation between the background portion and latent image portion increased, and the latent image of the characters “COPIED” stands out.
Therefore the second problem is that an attempt to increase the output density by increasing the screen ruling of the background portion formed by micro dots fails, and rather diminishes the concealment capability on the latent image in the original, because of the high screen ruling of the background portion and the major difference of the screen ruling between the latent image portion and the background portion.